2012-08-20 | Autoglass

Lamination defects typically represent no more than 1-2 per cent waste from the production of laminated windshields. These figures are not particularly high, but the total cost of waste created by lamination defects internally and externally is such that lamination requires special attention in process development work.
The complex nature of lamination processes and lamination defects requires process parameters to be individually adjusted according to the raw materials used, product design and processing equipment.

Vacuum ring installation testing with a client

With SGE I have been have been lucky to work and study on topics involving lamination using different transparent interlayers in addition to regular PVB products. We have learned a lot about Thermoplastic Polyurethane (TPU) and Ethylene Vinyl Acetate (EVA) in Bullet Resistant Glass applications, and also worked with sound insulating PVB interlayers applied to windshields. In this reference I have found that it is easy to overlook the conditions that contribute in reduced lamination process performance. The lamination process has a key role in laminated glass production line performance, and its results and quality are directly linked with bending and pre-processing activities. To understand why lamination waste is so expensive I will review two typical lamination defects and thereafter describe how to improve lamination process performance by introducing the key parameters to be observed.

The lamination processDominating the lamination process for the production of windshields contains the following process cycle:1. PVB preparation-> 2. PVB Assembly -> 3. Vacuum conveyor (cold and hot)-> 4. AutoclaveThere are also existing alternatives such as vacuum boxes, but the processing principles remain. The entire process seems simple, however, following the basic guidelines of clean environment work and common processing parameters alone is not enough to maintain and improve defect rates. Without perfectly controlled processing parameters, the process can result in a high number of...

With SGE I have been have been lucky to work and study on topics involving lamination using different transparent interlayers in addition to regular PVB products. We have learned a lot about Thermoplastic Polyurethane (TPU) and Ethylene Vinyl Acetate (EVA) in Bullet Resistant Glass applications, and also worked with sound insulating PVB interlayers applied to windshields. In this reference I have found that it is easy to overlook the conditions that contribute in reduced lamination process performance. The lamination process has a key role in laminated glass production line performance, and its results and quality are directly linked with bending and pre-processing activities. To understand why lamination waste is so expensive I will review two typical lamination defects and thereafter describe how to improve lamination process performance by introducing the key parameters to be observed.

The lamination processDominating the lamination process for the production of windshields contains the following process cycle:1. PVB preparation-> 2. PVB Assembly -> 3. Vacuum conveyor (cold and hot)-> 4. AutoclaveThere are also existing alternatives such as vacuum boxes, but the processing principles remain. The entire process seems simple, however, following the basic guidelines of clean environment work and common processing parameters alone is not enough to maintain and improve defect rates. Without perfectly controlled processing parameters, the process can result in a high number of re-autoclaved and waste pieces due to lamination defects.Waste created at this stage of the production chain is particularly expensive due to materials, processing costs, work time and disposal, and, last but not least, any re-autoclaved pieces create workflow in an unnatural direction. All re-autoclaved pieces are especially damaging for productivity as the autoclave is generally the bottleneck in a typical laminated glass production line, due to limited capacity and long process cycles taking up to 3.5 hours (depending on equipment and production).

Lamination defectsBubbles, teardrops and wormsLamination defects typically represent approximately 1.0-2.0 per cent waste from production. In addition to dirt particles/internal defects between the glass sheets, the defect known as “bubbles” – the name relating to its shape and appearance – is probably the most common lamination defect. Air is a key factor in the creation of bubble defects. Bubbles, teardrops and worms all are an indication of excess residual air between the laminate[1].However, this lamination defect is controversial: the bubbles do not seem to appear each time a little air is left between the laminate or when gapping between the glass pairs is monitored. The defect can arise when excess residual air alone remains between the laminate, but even more so when a few contributing factors are existent. The fact that there are various lamination and autoclaving process parameters, equal conditions of pre-processing and bending that affect process performance and especially de-airing makes complete elimination difficult. Bubble defects appear in different sizes and locations. Typically the defect appears as bubble-areas at the very edges of the glass, while larger individual bubbles also appear both at the edges as well more towards to the middle section. Larger bubbles appearing at the very edge of glass can usually be fixed by means of a re-autoclave cycle, adding clips that help to seal the glass edges.There are, however, drawbacks in this curing method. The re-autoclaving cycle creates a contra-current work flow, increases production costs, and some re-worked parts have the tendency to introduce worm-like delamination a while after re-autoclaving. Furthermore, using clips is controversial as the pressure applied with the clips potentially alters the thickness of the laminate and introduces stresses. However, larger bubbles in the inner parts of the glass, typically between the glass corner and mould hinge line, can be impossible to fix. Typical bubble defects are visible right after autoclaving, but residual air can unify to create worm-like de-lamination caused by residual air left between the glass sheets, a phenomenon described as super-saturation[1].

Ice flowers and snowflakesThe introduction of acoustic PVB products created manufacturing challenges as these new products require more attention to process quality than traditional PVB products. With the new acoustic PVB with trilayer structure, glass manufacturers started to suffer from a new kind of lamination defect, which is, today, known as ‘ice flower’ or ‘snowflakes’. This is a rather fascinating defect, most likely because of its nature and appearance, but is especially difficult for manufacturers suffering from it. It is still a popular topic of discussion both internally and between glassmakers and their customers. This defect is complicated, because it is a delayed defect that will appear typically after exposure to changing climate conditions and usually no earlier than in field, therefore creating huge external waste costs.The development of what is known as ‘advanced laminated glazing’ introduced a sound insulating PVB interlayer that allows the reduction of glass thickness without increased noise penetrating the vehicle interior. With its trilayer structure, this new PVB product enables to save weight without compromising passenger comfort. The acoustic PVB interlayer applied in the laminated glass formulation does not differ from the traditional PVB interlayer in make-up, and, furthermore, does not have special requirements from the manufacturing process. This being said, with lamination of acoustic PVB, the likelihood of any manufacturing process deficiency in lamination works resulting in defects will be increased significantly. It is simply a less forgiving material when it comes to lamination process quality.

Key parameters to be observedPre-processingCutting size variations will cause severe difficulties in positioning of the glass pair during lamination. Faults in the positioning of glass pairs in PVB assembly will cause unwanted gapping between the glass sheets. Although the test results in relation to acceptable gapping tolerance are contradictory, it is clear that it can contribute to the creation of bubbles and delaminations. Furthermore, gapping cured with lamination adds to the natural manufacturing stresses of the windshield and potentially decreases final product quality. Gapping between the glass sheets can be caused by temperature difference between the glass sheets in bending or mould issues. The in-house tolerances for acceptable gapping vary and the gapping affect is very product dependent. Therefore recognition of maximum gapping tolerance used in unison with another shape related key parameter will give a better indication of bubble probabilities.There are also a few finer details in pre-processing which can affect lamination; as an extensive amount of separating powder can cause minor surface variations in the glass pair’s inner surfaces, reducing adhesion of the PVB and glass. Furthermore, poor washing water characteristics can also affect adhesion. Therefore, the hardness of the water should be monitored. Salts, for example, can cause large deviations where adhesion may even drop by several pummel units. Last but not least, tin and airside orientation of the float glass should be controlled as they can affect adhesion levels. Therefore the affect/relationship to PVB adhesion is important and has to be acknowledged, and the decided standard orientation must be maintained.

PVB preparationIf rolls of PVB are stored open, storage should also maintain the correct temperature and relative humidity. After cutting, the pre-cut PVB sheets (blanks) are cured by relieving the sheets in a flat position, which enables the stresses caused by storing in the roll – and also shaping/stretching – to be removed from the material. The relieving time for blanks depends on the material used and individual processing conditions.Large numbers of pre-cut PVB blanks are not recommended as extensive stacks can lead to complications in thickness and also blocking.Lamination process conditionsGlass pairs are cleaned individually, with special attention to the inner surfaces. Cleaning actions should be conducted so that the glass and cleaning devices are not subjected to static electricity generated during cleaning actions.After the glass sheets are cleaned, the glass temperature should be at an acceptable level to enable blank lay-up.Before assembly works, it is highly important to eliminate the processing of defected parts by inspecting the glass sheets carefully after cleaning. SGE training programmes provide further information regarding the technical aspects of best performing inspection stations. De-airing conveyorDe-airing is the most important step of lamination and the process quality is closely linked with the lamination process yield[2]. First make sure that the correct vacuum ring vacuum level is reached. After the vacuum is started the sandwich should be subjected to sufficient cold vacuum time to prevent premature edge seal. Cold vacuum should be maintained for a period of time and the larger the windshields are the longer cold vacuum time they require. It is important to notice that cold vacuum should remove the major part of the trapped air. After the sufficient cold vacuum period has been passed the laminate will be subjected to heating. The most common reason for excess air in the laminate is usually processing faults at the very beginning of the heat treatment (hot-vacuum), enabling the glass edge to seal too early. Therefore to ensure that no premature edge sealing takes place the hot de-airing phase heating should be conducted gradually and uniformly to relieve all excess air from the sandwich. Vacuum is to be maintained through the entire heating period. The required glass temperature and heating time is dependent on the raw materials and on the processing equipment used. The key factor here is to subject the materials to sufficient temperature for the PVB interlayer to soften, edges to be sealed and to create preliminary adhesion between the glass and PVB. After the glass has passed the heating section it should be cooled sufficiently before the removal of the vacuum rings, notice the cooling stage at the conveyor. Removing the vacuum rings from cooled glass prevents edge seals from opening. At this stage ensure that good opacity has been reached after the de-airing conveyor process = glasses are almost transparent. Cloudiness of the edges is an indication of issues in edge sealing.Autoclaving processThe autoclaving process is the final real treatment process of a typical windshield that can significantly affect lamination and end product quality as there is a dependency on adhesive bond strength and autoclave temperature. The process temperature and pressure curves can be controlled in great detail with modern autoclaves. Correct temperature and pressure set points are required to gain perfect results. Process set point issues can result in bubbles appearing days after the process has taken place.

SummaryThe importance of the lamination process excellence is evident. Lamination experts agree that the de-airing step is the most important one in determining yield[2]. Since air is a key-factor in defect formation, all processing parameters and conditions should aim at minimizing the amount of residual air in the laminate. Naturally we must not neglect the contributing factors. Processing conditions and used materials vary; therefore the followed process guidelines must be adapted to individual situations. Lamination process conditions must be well controlled to enable adjustments and identification of process variations when defects appear. Lamination processes include various minor details that potentially affect glass performance and quality. Many of these details are linked with bending and pre-processing quality. Mastering lamination process parameters is essential. SGE provides expert advisory conducted by individuals with in-depth expertise and practical experience in solving production challenges related to heat-treatment processes: tempering, bending, lamination and also mould tooling.All graphics, photographs, and text appearing in this article belong to Safety Glass Experts International Oy Ltd. Redistribution or commercial use is prohibited without express written permission.References1 Dr. Bert C.Wong Shattering Old Myths about Defect Formation in Laminated Glass. Part II2 Luc.A.Moeyersons: Edge-performance testing of automotive laminated glassThe AuthorMika Eronen is a Finnish safety glass professional. Mika started his career in the glass industry at Pilkington Finland in 1999. Working with the most complex bus windshields in the markets, he has developed a good knowledge on windshield manufacturing. In 2004, taken by international challenges, Mika toured globally carrying out new hot-end machinery start-ups, customer training, and consultation projects, still developing his skills further in the art of sag bending process/technology and production of windshields. Today Mika has finished his professional specialization studies and is a founder and owner of Safety Glass Experts International Ltd. Mika has developed a passion to write about safety glass processing and, on top of his routine technical article publications he is soon to finish the first ever complete technical guide book about windshield manufacturing called “The Secrets of Windshield Manufacturing”.